Conventionally, it is known a display device such as a head-up display (hereinafter referred to as “HUD”) which visualizes images as virtual images (for example, Patent References 1 and 2). Normally, by the HUD, real images formed by a real image display device (i.e., images on an LCD display or images projected on a screen by a projector) is recognized by a driver as virtual images by the use of a half mirror, called as a combiner, disposed ahead of a visual field of the driver. Thus, the driver can visually recognize meters, navigation information and the like, in a manner superimposed on a front view, while keeping his or her visual line ahead without dropping the visual line.